Squaraine dyes are known to possess photoconductive and semiconductive properties. These features have made them very attractive for various industrial applications such as xerographic photoreceptors, organic solar cells, optical recording media, antihalation dyes and acutance dyes.
The general structure of squaraine dyes is shown in dye 1. In this ##STR1## structure, R is generally N or O while R.sub.1, R.sub.2, R.sub.3, and R.sub.4 can be H, organic substituents or together form another aromatic ring.
The synthesis of squaraines (dye 1) wherein R.sub.1 and R.sub.2 together form a second phenyl ring has been reported. Bello describes squaraine synthesis in n-butanol and toluene with azeotropic removal of water (K. A. Bello, S. N. Corns, and J. Griffiths, J. Chem. Soc., Chem. Commun., 1993, 452-454). U.S. Pat. Nos. 5,380,635 and 5,360,694 describe the synthesis of squaraine dyes in the same manner. None of these references attempts to describe optimal conditions for preparation, nor do they comment on preferred synthetic procedures.
Other synthetic procedures for squaraine dyes have been reported. These methods describe the preparation of squaraines (dye 1 ) wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are H or simple organic substituents and do not form a second aromatic ring. These dyes were first reported by H. Sprenger and W. Ziegenbein (Angew. Chem. Internat. Ed. Engl., 5, 894 (1966)). Their procedure consists of heating in benzene and n-butanol, with azeotropic removal of water.
K. Law, F. C. Bailey, and L. J. Bluett (Can. J. Chem., 64, 1607-1619 (1986)) describe the synthesis of this dye 1 (R=dialkylamino, R.sub.1 =H or a ring to R, R.sub.2 =H, methyl, F, ethyl, or methoxy, R.sub.3 =H, R.sub.4 =H) in either toluene or benzene with butanol, with azeotropic removal of water. They note the increase in solubility of dyes with longer alkyl chains and the decrease in the isolated yields of these dyes which "might be attributable to secondary reactions of squaraines with N,N-dialkylanilines in the reaction mixture". They also noted that "in controlled experiments that squaraines react with N,N-dialkylanilines to form colorless products", which they do not identify. They do not suggest any cures for these synthetic difficulties. Their yields of the soluble dyes were less than 9%. Column chromatography was required to purify these dyes. On the other hand, dyes with shorter alkyl chains precipitated directly from the reaction mixture and after simple filtration and solvent washing were said to be analytically pure. Yields for these less soluble dyes ranged from 9.5 to 60%.
K. Law and F. C. Bailey looked further into the synthetic procedure (Can. J. Chem., 64, 2267-2273 (1986)). They contrasted two synthetic procedures, one they referred to as the "acid route" and the other the "ester route." The acid route is the traditional method and involves heating squaric acid and N,N-dialkylaniline in azeotropic solvent, including an alcohol. The ester route involves heating a diester of squaric acid and an N,N-dialkylaniline in an alcoholic solvent, and requires additional water. Heating di-n-butyl squarate and N,N-dimethylaniline in freshly dried n-butanol gave no dye. Incremental addition of water in the presence of an acid (sulfuric, oxalic, trichloroacetic, or toluenesulfonic) resulted in increased yields of dye. The highest yields were obtained in water-saturated n-butanol. Increasing the concentration of the acid (with water present) resulted in first an increase, and then a decrease in the dye yield. They suggested these results indicate that the reactive intermediate in the reaction is the half ester of squaric acid, 2. Too much acid protonates some of the N,N-dialkylaniline, reducing its reactivity. ##STR2##
Law and Bailey also found that in the "acid route", no squaraine dye is formed if a non-hydroxylic solvent, or a secondary or tertiary alcohol is used as solvent. This is ascribed to the slower rate with which such alcohols can esterify squaric acid. They also noted that the necessity of having water in the reacting solvent in the ester route is in contrast to the acid route where water is removed continuously by an azeotropic solvent during the course of the reaction.
Law and Bailey also examined the effect of the alcohol in the "ester route." Short chain alcohols were found to give higher yields than longer chain alcohols: dimethyl squarate gave 52%, di n-propyl-47%, di n-butyl-45%, and di n-heptyl-27%. This they ascribe to an increased steric effect retarding the initial hydrolysis to 2. They also demonstrated that the ideal amount of N,N-dimethylaniline to be used is the expected 2:1 molar ratio to squarate.
Law and Bailey examined the role of water in the "ester route" by closely monitoring the boiling point of the water/saturated n-butanol reaction. They found that the initial boiling point (96.degree. C.) slowly increased over 8 hours to 118.degree. C. as the water/n-butanol azeotrope lost water (due to both hydrolysis of the ester and azeotropic removal from the reactor) and the medium became dry n-butanol. This removal of water from the system drives the reaction to product.
Law and Bailey further investigated the rate of addition of the N,N-dialkylaniline on the reaction. They added the N,N-dialkylaniline very slowly (over 6 to 8 hours) to the reaction mixture. They proposed that this suppresses side reactions and encourages the aniline to react with 2 (which is slowly formed from the dialkyl squarate). They said that the slow addition is especially important with highly reactive anilines (such as N,N-dimethyl-3-hydroxyaniline). Yields decreased by 30 to 50% when the aniline was added in a single batch at the beginning of the reaction.
In J. Imaging Sci., 31, 172-177 (1987), K. Law and F. C. Bailey found that the squaraine dyes prepared by their "ester route" contained fewer impurities than the same dyes made by the "acid route." This resulted in better xerographic properties for the "ester route" samples.
Further work by K. Law and F. C. Bailey (Dyes and Pigments, 9, 85-107 (1988)), examined the synthesis of N-benzyl substituted squaraine dyes. In this case, they compared the "acid route" at 70 torr in either n-butanol and toluene, or in n-heptanol. Higher yields were found using n-heptanol, but at the expense of lowered purity. Impurities of structure 3 were found in the n-heptanol reactions. Also, some dyes could only be prepared in n-heptanol, no yield was obtained in butanol. ##STR3##
Symmetrical and unsymmetrical squaraine dyes have also been produced by an alternative route (See K. Law and F. C. Bailey, J. Chem. Soc., Chem. Commun., 1990, 863-864; K. Law and F. Court Bailey, J. Chem. Soc., Chem. Commun., 1991, 1156-1158; K. Law and F. C. Bailey, J. Org. Chem., 57, 3278-3286 (1992)) and are summarized in the reaction scheme shown below. Here the intermediate aryl hydroxy cyclobutenedione 4 is prepared by a ketene-olefin cycloaddition. The dye is then prepared in a separate step. This synthetic scheme is covered in U.S. Pat. Nos. 4,886,722; 4,922,018; and 5,030,537. ##STR4##
U.S. Pat. No. 4,524,219 (1985) (K. Law) is an example of the ester route and covers the reaction of a dialkylsquarate with an aniline in an alcohol with an acid catalyst. Water is not specifically mentioned in this patent, although the alcohol is referred to as "dry".
U.S. Pat. No. 4,525,592 (1985) (K. Law and F. C. Bailey) covers the same ester route as U.S. Pat. No. 4,524,219, but this time the examples indicate that water was added to the solvents.
U.S. Pat. No. 4,524,220 (1985) (K. Law) covers the reaction of squaric acid in n-butanol and benzene with an aniline, but with an added aliphatic amine. The resulting dyes are said to have improved photoconductive properties. The role of the added amine is not speculated upon.
U.S. Pat. No. 4,523,035 (1985) (J. F. Yanus) describes the use of a higher alcohol (such as heptanol) at reduced pressure with or without an acid catalyst to prepare squaraine dyes. The advantages stated are that the water separates more readily from heptanol than from butanol, that the reaction can be more readily scaled up, that competitive reactions are reduced, and that diester formation is reduced. This patent states that the butanol reactions cannot be scaled up beyond a batch size of 0.5 mole whereas the higher alcohol reactions are scaleable.
In summary, soluble squaraine dyes are known to be quite unstable in normal reaction mixtures leading to extensive decomposition during the synthesis of the squaraine system, as indicated by K. Law, F. C. Bailey, and L. J. Bluett (Can. J. Chem., 64, 1607-1619 (1986)). The squaric ester route described above by K. Law and F. C. Bailey (Can. J. Chem., 64, 2267-2273 (1986)), required lower alcohols, like propanol, for high yield and the exact balance of water in the reaction was critical. This type of process would be very difficult to scale up. It should be noted that there is no indication in the related art regarding the beneficial effects of additional water in the "acid route". Typically, the preparation of soluble dyes requires extensive purification of the final product by solvent extraction, recrystallization, and/or chromatography. These steps are time consuming, expensive and can generate hazardous waste.
A need exists for a simple cost effective method for the production of soluble squaraine dyes. Dyes of this type when prepared by known methods are difficult to scale up and isolate in good yield and purity.